Even though, above and in the following, paper and paper machines are spoken of, in the present invention, these terms also include board and board machines.
The highest web speeds in paper machines are today up to an order of about 25 meters per second (mps) and slightly higher, but before long, a web running speed in the range of 25-40 meters per second will be commonly used. In such a case, a bottleneck for the runnability of a paper machine will be the dryer section, whose length with the prior art multi-cylinder dryers would also become intolerably long. If it is imagined that a present day multi-cylinder dryer were used in a newsprint machine at a web speed of about 40 mps, it would include about 70 drying cylinders (.phi..apprxeq.1800 mm), and its length in the machine direction would be about 180 meters. In such a case, the dryer section would comprise about 15 separate wire groups and a corresponding number of draws over group gaps. It is probable that, in a speed range of 30-40 mps, the runnability of normal prior-art multi-cylinder dryers is no longer even nearly satisfactory, but web breaks would occur quite often lowering the efficiency of the paper machine.
In a speed range of 30-40 mps and at higher speeds, the prior art multi-cylinder dryers would also become uneconomical because the cost of investment of an excessively long paper machine hall would become unreasonably high. It can be estimated that the cost of a paper machine hall is at present typically about 1 million FIM per meter in the machine direction.
In the prior art, in multi-cylinder dryers of paper machines, twin-wire draw and/or single-wire draw is/are employed. When employing twin-wire draw, a group of drying cylinders comprises two closed (endless) wires, fabrics or belts which press the web one from above and the other one from below against heated cylinder faces of drying cylinders arranged in rows. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws which are susceptible to fluttering and may cause web breaks, in particular when the web is still relatively moist and, therefore has a low strength. For this reason, in recent years, ever increasing use has been made of the single-wire draw in which each group of drying cylinders includes only a single closed (endless) drying wire on whose support the web runs through the entire group so that the drying wire presses the web on the drying cylinders against the heated cylinder faces thereof, whereas on the reversing cylinders or rolls between the drying cylinders, the web remains at the side of the outside curve and is subjected to negative pressure as it runs over the reversing cylinders in order to maintain the web on the wire. Thus, in single-wire draw, the drying cylinders are arranged outside the wire loop, and the reversing cylinders or rolls are arranged inside the wire loop.
It is known to those skilled in the art that if paper is dried one-sidedly, the result is a tendency of curling of the sheet. For example, when paper is dried by means of normal groups with single-wire draw from the side of its bottom face only, the drying is asymmetric and if such asymmetric drying is extended over the entire length of the forward dryer section, the drying takes place so that first the bottom-face side of the paper web is dried and, when the drying makes progress, the drying effect is also extended to the side of the top face of the paper web. Under these circumstances, the dried paper is usually curled and becomes concave, when viewed from above.
One parameter that illustrates the drying capacities of the prior art multi-cylinder dryers is the amount of water evaporated in the dryer section per unit of length and width, i.e., per floor area covered by the web to be dried, within a unit of time. In prior art multi-cylinder dryers, this parameter is typically in the range of 50-80 kilograms of H.sub.2 O per square meter per hour.
It is known from the prior art to use various impingement-drying/through-drying units for evaporation drying of a paper web, which units have been employed in particular in the drying of tissue paper. With respect to this prior art, reference is made, by way of example, to the following patent literature: U.S. Pat. Nos. 3,301,746, 3,418,723, 3,447,247, 3,541,697, 3,956,832 and 4,033,048, Canadian Patent No. 2,061,976, West German Patent Application Nos. DE-A-22 12 209 (corresponding to U.S. Pat. No. 3,816,941) and DE-A-23 64 346 (corresponding to U.S. Pat. No. 4,033,049, European Patent Application No. EP-A2-0 427 218), Finnish Patent Nos. 57,457 (corresponding to Swedish Patent Application No. 7503134-4) and 87,669 (corresponding to U.S. Pat. No. 5,383,288), and Finnish Patent Application No. 931263 (corresponding to U.S. Pat. No. 5,495,678 and European Patent Application No. 0 620 313-A1).